Field of the Invention
This invention relates to an internal combustion engine, and more particularly to an internal combustion engine that includes a piston on which a film (anodic oxide film) is formed by an anodic oxidation treatment.
Background Art
An anodic oxide film that includes a porous layer that is formed by subjecting a top face of a piston base material made of an aluminum alloy to an anodic oxidation treatment, and a film layer that is formed by plasma spraying of a Y2O3-stabilized ZrO2 powder onto the surface of the porous layer is known, as disclosed, for example, in Japanese Patent Laid-Open No, 2012-72745. The porous layer has a large number of pores that are formed during the course of the anodic oxidation treatment, and the film layer is provided so as to seal the pores. Because an anodic oxide film having such a structure has a lower thermal conductivity and a lower thermal capacity than a conventional ceramic-based insulation film, the anodic oxide film is useful for reducing the cooling loss of an internal combustion engine. Further, according to the aforementioned Japanese Patent Laid-Open No. 2012-72745, a sealing treatment is performed after a convex-concave pattern is formed on the surface of the porous layer to improve a bonding property with respect to a coating layer, and thereafter the coating layer is formed by a finishing process. As a result, roughness in the coating layer that is produced by formation of the convex-concave pattern is smoothed.
Japanese Patent Laid-Open No. 2012-122445 discloses an anodic oxide film in which a metal such as platinum is carried inside pores of a porous layer formed by subjecting a top face of a piston base material to an anodic oxidation treatment. Unlike Japanese Patent Laid-Open No. 2012-72745, a sealing treatment is not performed on the porous layer according to Japanese Patent Laid-Open No. 2012-122445. However, according to the anodic oxide film described in Japanese Patent Laid-Open No. 2012-122445, soot generated inside a combustion chamber can be oxidized and purified by a catalytic action of the metal.
Japanese Patent Laid-Open No. 61-142320 discloses that a heat insulating material such as ZrO2 is coated on or adhered to side faces of a combustion chamber that correspond to a dead volume of the combustion chamber in the vicinity of the top dead center of a direct injection type or divided chamber type diesel engine, that is, a top face and an upper side face of a piston, a non-sliding face of an upper portion of a cylinder liner, and an outer circumferential portion of a face on the combustion chamber side of the cylinder head. The heat insulating material described in Japanese Patent Laid-Open No. 61-142320 is a so-called “ceramic-based heat insulating material” and not an anodic oxide film.